A dual-focus objective lens for an optical pickup which is compatible with both 0.6 mm and 1.2 mm disks is compensated in its aberration with respect to a light spot on a recording plane of the loaded disk and is composed of a first lens and a second lens. The loaded positions of the 0.6 mm disk and...http://www.google.fr/patents/US5734512?utm_source=gb-gplus-shareBrevet US5734512 - Dual-focus objective lens of optical pickup
